Elongation

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To determine the sign of elongation the first order or 530 nm compensating filter is used with crossed polars. The filter adds retardation to the sample. It is made from an anisotropic material. The z direction is the high refractive index or "slow" direction. If the longer wavelength polarization color appears when the long direction of the crystal is parallel to the z axis then the elongation is positive (figure A). If it is parallel to the x axis (ie., blue in figure B) then the elongation is negative.